Continous casting mould

ABSTRACT

An open-ended mold for casting metal strands with a thin slab cross section, the mold having broad-side walls which form a central widening region which extends from the mold inlet edge to the mold outlet edge and at the outlet edge opens out in a rectangular outlet cross section. To prevent to a great extent the strand from coming away from the broad-side walls, the contour of the mold cavity in the widening region is formed, as seen in sectional planes taken at right angles to the longitudinal axis of the mold, by curves which at least in partial regions form clothoids or clothoidal curves, within which sudden changes in curvature (Δk) are minimized or reduced to zero. The curvature reduces to flat at the mold outlet edge. Other cavity surface features are disclosed.

[0001] The invention relates to an open-ended mold for a continuouscasting installation, in particular an open-ended steel casting mold,for casting a metal strand with a thin slab cross section with a maximumthickness of 150 mm, with a mold cavity formed by cooled broad-sidewalls and narrow-side walls, the mutually opposing broad-side wallsforming a central widening region, which extends from the mold inletedge to the mold outlet edge and ends there in a rectangular outletcross section of the open-ended mold and which goes over in thedirection of the mutually opposing narrow-side walls into narrow-sideadjusting regions.

[0002] Open-ended molds of this type are referred to in practice as“funnel molds” and are described with various configurations of thefunnel-shaped widening region in the literature. The many embodimentsthat have been derived from the original Rossi mold (DE-C 887 990) andthe first trial mold with a funnel-shaped widening region (EP-B 149 734)reflect the problems in the forming of the strand shell and the shapingof this strand shell in the spatially curved widening region.

[0003] On the inlet side, the known open-ended molds have in the regionof the meniscus a spindle-shaped cavity cross section, which isconstricted continuously in the direction of the narrow-side walls tothe size of the strand to be cast and is made to revert to the size ofthe desired strand thickness in the strand drawing-out direction orcasting direction, either still within the length of the mold ordirectly in the outlet cross section or in the downstream backup andguiding roll stand of the continuous casting installation.

[0004] By this configuration it is possible to produce in such a moldsteel strands which, with customary slab widths, have strand thicknessesof less than 150 mm, preferably however 30 to 100 mm, and at the sametime to introduce melt into the open-ended mold with submerged castingnozzles which are thicker than the target thickness for the cast strand.

[0005] EP-A 1 002 599, DE-A 39 07 351 and EP-A 552 501 already discloseopen-ended molds in which the funnel-shaped widening region is formed insectional planes taken at right angles to the longitudinal axis of themold by arc portions which replicate the concave-convex-curved spindleform. In particular at the points of inflection at which arcs of acircle with radii deviating from one another merge in line with oneanother, there are great sudden changes in radius and curvature. Atthese points of discontinuity, the strand shell comes away from the moldwall, whereby inhomogeneous temperature conditions occur in the strandshell and, as a result, there is uneven strand shell development. As afurther consequence, this leads to thermal stresses between neighboringstrand zone and increased loads, and also under some circumstances todamage of the strand shell.

[0006] EP-A 552 501, EP-A 909 597 and DE-A 39 07 351 disclose forexample open-ended molds in which the contour of the funnel-shapedwidening region change in the longitudinal direction, i.e. alongsectional planes parallel to the longitudinal axis of the mold, abruptlyor with a great change in curvature. This usually applies to molds inwhich the funnel-shaped widening region already ends at a relativelygreat distance from the mold outlet edge. Apart from the disadvantagesalready described above of sudden or great changes in radius andcurvature of the curves describing the widening region, additional loadson the strand shell are produced by the oscillation of the mold. Thelocally intensified knocking of the mold on the funnel-shaped taper ofthe strand brings about additional impairment of the strand shell inthis region.

[0007] An open-ended mold of the type described at the beginning, with awidening region which extends from the mold inlet edge over the entirelength of the mold and is only made to revert to the rectangular crosssection of the strand to be cast in the mold outlet cross section isalready known from DE-C 35 01 422. It is also the case with this moldthat the contour of the widening region is formed in sectional planestaken at right angles to the longitudinal axis of the mold by arcs of acircle adjoining one another in line and likewise has the disadvantagesdescribed above. The shortening of the circumferential length of thestrand inevitably occurring as a result of the reversion of the wideningregion is counteracted by a corresponding adjustment of the narrow-sidewalls and a distortion of the strand shell is already avoided in thisway.

[0008] The object of the present invention is therefore to avoid thedescribed disadvantages of the known prior art and to propose anopen-ended mold with which the contour of the mold cavity is optimizedin such a way that coming away of the strand from the mold wall is to agreat extent prevented from the meniscus to the mold outlet edge, andconsequently homogeneous forming of the strand shell in the mold isensured.

[0009] This object is achieved according to the invention by the contourof the mold cavity in the widening region being formed in sectionalplanes taken at right angles to the longitudinal axis of the mold bycurves which at least in partial regions form clothoids or clothoidalcurves, within which the sudden changes in curvature are minimized orreduced to zero. Extensive avoidance of sudden changes in curvaturecreates the best conditions for optimum forming of the strand shell.

[0010] The described advantage of avoiding the strand shell coming awayis equally obtained if curve portions formed approximately or exactly byarcs of a circle or straight lines are joined by curve portions formedby clothoids or clothoidal curves and sudden changes in curvature ofthese curve portions at their transitional points are minimized orreduced to zero.

[0011] A preferred configuration in the central region of the wideningregion is obtained if the contour of the mold cavity in the wideningregion is formed in sectional planes taken at right angles to thelongitudinal axis of the mold by curves running symmetrically inrelation to the narrow-side walls with a convex-concave profile, which,starting from the center of the mold cavity contour with only a small orno curvature, go over into a region with constantly increasingcurvature. This effectively counteracts tendencies of the strand shellto come away in the central region.

[0012] Favorable conditions are obtained thereby if the curvature (k) inthe center of the mold cavity contour meets the condition

k≦0.3*(4H)/(H ² +L ²),

[0013] where k=curvature (mm)

[0014] H=half the maximum mold cavity widening (mm)

[0015] L=half the width of the widening region B₁ (mm)

[0016] An improvement in the forming of the strand shell and guiding ofthe strand shell along the broad-side walls is obtained if the suddenchanges in curvature at the transition from the central widening regionto the narrow-side adjusting regions and at the location of theinflectional tangents in the curve profile are minimized, preferablytend toward zero. This produces favorable conditions if the suddenchanges in curvature (Δk) at the transition from the central wideningregion (11) to the narrow-side adjusting regions (12, 13) meet thecondition Δk≦0.5*(4H)/(H²+L²), or if the sudden changes in curvature(Δk) at the location of the inflectional tangents (25) meet thecondition Δk≦0.4*(8H)/(H²+L²).

[0017] The coming away of the strand shell from the broad-side wall ofthe mold is to a great extent avoided if the contour of the mold cavityin the widening region is formed in sectional planes taken parallel tothe longitudinal axis of the mold and the narrow-side walls by straightlines between the mold inlet edge and the mold outlet edge. This linearformation of the central widening region in the longitudinal directionof the mold has the effect that a change in curvature conducive to thestrand shell coming away only occurs at the mold outlet. There, however,no adverse effect caused by an inhomogeneous temperature distribution isto be expected, since in this region the effect of the direct spraycooling on the cast strand commences.

[0018] Critical regions with regard to the coming away of the strandshell from the broad-side wall of the mold are formed by the maximumwidening, the transition from the central widening region to theadjoining narrow-side adjusting regions and the regions of the moldcavity that have points of inflection. Here, minimizing of the comingaway of the strand shell is achieved on the one hand by the transitionfrom the central widening region and the narrow-side adjusting regionlaterally adjoining the latter being formed by a straight line which, ina sectional plane taken parallel to the longitudinal axis of the moldand the narrow-side walls, joins the mold inlet edge and the mold outletedge, on the other hand by the locations of the inflectional tangents inthe contour of the mold cavity with sectional planes taken at rightangles to the longitudinal axis of the mold forming straight linesbetween the mold inlet edge and the mold outlet edge which lie in asectional plane taken parallel to the longitudinal axis of the mold.

[0019] In keeping with operational practice, it is advantageous if thestraight lines are arranged inclined by the extent of strand shrinkage.The inclination corresponds in its spatial orientation and order ofmagnitude to the values customary for a taper setting of the narrowsides. This measure is intended to avoid the strand shell having to runthrough a transition from a parallel wall region into the wideningregion in the strand drawing-off direction.

[0020] In order to meet special solidifying and shrinking conditions ofparticular grades of steel, it is expedient if the contour of the moldcavity in the widening region has in sectional planes taken parallel tothe longitudinal axis of the mold and the narrow-side walls a curved,preferably parabolic, profile between the mold inlet edge and the moldoutlet edge, in dependence on the solidifying conditions of the metal tobe cast. This also applies especially to the regions of the transitionfrom the central widening region to the adjoining narrow-side adjustingregions and the locations of the points of inflection or inflectionaltangents, if appropriate with allowance for a shrinkage-dependentinclination.

[0021] Further favorable influences on the quality of the strand to becast are obtained if the radii of curvature of the contour of the moldcavity in the widening region in sectional planes taken at right anglesto the longitudinal axis of the mold become greater between the moldinlet edge and the mold outlet edge in the direction of the mold outletedge and are infinite at the outlet edge. With an overall mold conceptin mind, it is expedient if the broad-side walls in the narrow-sideadjusting regions are arranged essentially parallel to one another andthe narrow-side walls are convexly formed in dependence on the shrinkingbehavior of the metal respectively to be cast.

[0022] Further details, features and advantages emerge from theexplanations and representations which follow of an exemplary embodimentnot restricting the extent of protection.

[0023] In the drawings:

[0024]FIGS. 1a, 1 b, 1 c show a schematic representation of theopen-ended mold according to the invention in a plan view (view Z),front view and side view (section A-A) according to one possibleembodiment of the invention,

[0025]FIG. 1d shows a further embodiment of the open-ended moldaccording to the invention in side view (analogous to section A-A inFIG. 1c),

[0026]FIG. 2 shows the open-ended mold according to the invention in alongitudinal section with submerged casting nozzle and cast strand,

[0027]FIG. 3 shows the coming-away behavior of the strand from thebroad-side wall when there are great changes in curvature in a centralwidening region of a rectangular shape in a continuous casting moldaccording to the prior art,

[0028]FIG. 4 shows the coming-away behavior of the strand from thebroad-side wall when there are great changes in curvature in a centralwidening region of a trapezoidal shape in a continuous casting moldaccording to the prior art,

[0029]FIG. 5 shows the coming-away behavior of the strand from thebroad-side wall when there are minimized changes in curvature in acentral widening region of a rectangular shape in a continuous castingmold according to the invention,

[0030]FIG. 6 shows a comparison of the contour of the central wideningregion and the associated curvature profiles in the case of the moldaccording to the invention and a mold according to the prior art.

[0031] An open-ended mold of the type according to the invention isrepresented in FIGS. 1a to 1 c in a schematic representations. Itcomprises two mutually opposing broad-side walls 1, 2 and two likewisemutually opposing narrow-side walls 3, 4, which can be adjusted betweentwo positions B, B1 in the sense of a format adjustment. Bounded by thebroad-side walls 1, 2 and the narrow-side walls 3, 4, between the moldinlet edges 5 and the mold outlet edges 6, is the mold cavity 7 of theopen-ended mold, in which a cast strand, comprising a strand shell 9 anda liquid core 10, is formed between a meniscus (liquid metal level) 8and the mold outlet edge 6, as illustrated in FIG. 2.

[0032] The broad-side walls 1, 2 have a central widening region 11,which is bounded essentially by the narrowest adjusting region B1 of thenarrow-side walls 3, 4 and goes over on both sides into the narrow-sideadjusting regions 12, 13. These narrow-side adjusting regions are formedby mutually plane-parallel wall parts of the broad-side walls 1, 2 andextend from the mold inlet edge 5 to the mold outlet edge 6. As adeparture from this embodiment, it is quite possible for the broad-sidewalls to form planar surface areas arranged such that they converge inthe strand drawing-off direction or the direction of the narrow-sidewalls in the region of the narrow-side adjusting regions.

[0033] The central widening region 11 has at the mold inlet edges 5 across section which narrows in the form of a spindle to the narrow-sideadjusting regions 12, 13. It is provided in its central region with awidth adequate for receiving a submerged casting nozzle 14 and isreduced toward the narrow-side adjusting regions 12, 13 to the thicknessD of the metal strand to be cast by curves 15 with a curvature profilethat is to a great extent free of any sudden changes. These curves 15comprise for example arcs of a circle 16, 17, which are joined in lineby clothoids 18, 19, 20, 21 or clothoidal curves and the curvatureprofile of which runs without any abrupt changes or with abrupt changesminimized.

[0034] The central widening region 11 is continuously constrictedbetween the mold inlet edge 5 and the mold outlet edge 6 and is reducedat the mold outlet edge 6 to the size of a rectangular outlet crosssection 22, which corresponds to the desired casting cross section. Thecontour profile of the central widening region 11 between the mold inletedge 5 and the mold outlet edge 6 is formed, in relation to a sectionalplane which is taken parallel to the longitudinal axis 23 of the moldand parallel to the narrow-side walls 3, 4, by straight lines 24 a, 24b, . . . 24 n. This linear configuration of the central widening regionin the casting direction ensures freedom from deformation in thevertical strand drawing-off direction. This definition of the sectionalplane is based on the assumption that the narrow-side walls 3, 4 arearranged plane-parallel to one another and there is therefore no taperadjustment.

[0035] Consequently, the end faces of the planar narrow-side walls areoriented parallel to the longitudinal axis of the mold. A taperadjustment of the narrow-side walls that is provided in practicetherefore does nothing to change the orientation of the straight linesdescribed. The central widening region 11 can also be constrictedbetween the mold inlet edge 5 and the mold outlet edge 6 by a curve witha parabolic profile 26. (FIG. 1d)

[0036]FIG. 3 illustrates results of numerical investigations on afinite-element basis in a half-section of a broad-side wall between themeniscus and the mold outlet edge by an example of an open-ended moldaccording to the prior art with a central widening region which is of arectangular shape in an end-on view and extends in the transversedirection between the line I, which corresponds to the longitudinal axis(23) of the mold in FIG. 1a, and the narrow-side adjusting region, andin the longitudinal direction between the meniscus and the line A. Thelight areas show regions at which the coming away of the strand shellfrom the broad-side wall is particularly pronounced. In these regions, agreat change in curvature occurs in the contour of the central wideningregion. At these points, inhomogeneous temperature conditions andreduced strand shell development are to be expected. A distinction canbe made here between essentially three regions in the transversedirection, indicated by the vertical lines I, II, III, and a region inthe longitudinal direction, indicated by the horizontal line A.Particularly at risk is the region with the greatest widening in themold (line I), followed by two regions (lines II, III) at the curvetransitions toward the edge of the mold. In the longitudinal directionthere follows at the end of the funnel an extended region (line A) withdiminishing contact of the strand shell with respect to the broad-sidewall. This may be regarded as a serious disadvantage of funnel molds inwhich the funnel ends inside the mold.

[0037]FIG. 4 shows analogous results with respect to a funnel mold witha V-shaped constriction of the widening region between the meniscus andthe mold outlet edge, as is likewise known from the prior art. Thestructural composition of the representation corresponds to FIG. 3. Inthe case of this type of mold too, great regions with disturbed contactbetween the strand shell and the mold wall occur. This is attributableto the V-shaped funnel edge (line V), which lies obliquely in relationto the casting direction and which the strand/strand shell must pass.The associated changes in curvature in the longitudinal direction causethese disturbances of the contact behavior.

[0038]FIG. 5 shows the results of the numerical investigation on afinite-element basis with reference to an open-ended mold according tothe invention. The structural composition of this representationcorresponds in turn to that of FIG. 3. As a result of the optimizationaccording to the invention of the contour of the mold cavity of themold, light regions with disturbed contact between the strand shell andthe broad-side wall now occur in the direct vicinity of the mold outletedge, where no adverse effects on the forming of the strand shell occurany longer.

[0039] A comparison of the contours of the mold cavity in the wideningregion in the case of a funnel mold according to the prior art and amold according to the invention in a sectional plane taken at rightangles to the longitudinal axis of the mold is represented in FIG. 6.Represented in the upper half of the figure is the contour profile ofthe mold width, starting from the center of the mold (longitudinal axisof the mold), and the lower half of the figure shows the assignedcurvature profile over the mold width. The contour profile according tothe prior art, represented by dotted lines, is formed by two arcportions merging one into the other and ends in a straight line whichidentifies the narrow-side adjusting region. The associated curvatureprofile, represented by a dotted line, jumps at the contact point of thetwo arc portions (point of inflection 25′) from an until then constantnegative value to a constant positive value. Occurring here is themassive point of discontinuity in the curvature profile that wasdescribed as disadvantageous and contributes significantly to the comingaway of the strand shell. Conditions with the same effect occur at thetransition from the arc to the straight line at the transition to thenarrow-side adjusting region. Here, the curvature jumps at a point ofdiscontinuity back to the value zero. In comparison with this,represented by the solid line is the contour profile according to anembodiment of the invention, which, starting from the center of themold, is formed by a clothoid 18 or clothoidal curve with a constantlyincreasing curvature, which is adjoined by an arc portion 16 with aconstant curvature. This is adjoined by a transitional portion which isformed by two clothoids 19, 20 or clothoidal curves and in the profileof which the curvature is changed continuously from a negative maximumvalue to a positive maximum value as it passes through a point ofinflection 25. There further adjoins an arc portion 17 with a positivecurvature value, which is then transformed by a clothoid 21 orclothoidal transition curve to the curvature value zero of a straightline. The avoidance of sudden changes in curvature is provided by thiscontour profile.

[0040] Instead of the arc portions in the contour profile that arespecified in this exemplary embodiment, curve portions formed toapproximate arcs of a circle may also be used. In the region of maximumwidening in the widening region, a curve portion formed by a straightline or approximately by a straight line may be provided. All thesemodifications are within the extent of protection of the invention.

1. An open-ended casting mold for a continuous casting installation forcasting a metal strand with a thin slab cross section the moldcomprising: a mold cavity formed by mutually opposing, spaced apart,cooled broad-side walls and of mutually opposing, spaced apart,narrow-side walls between the broad-side walls, the mold having a moldinlet edge and a mold outlet edge; the mutually opposing broad-sidewalls forming a central widening region between the broad-side walls,and which extends from the mold inlet edge to the mold outlet edge andthe widening region ending at the mold outlet edge in a rectangularoutlet cross section of the open-ended mold; the widening regionextending in the directions toward and narrowing toward the mutuallyopposing narrow-side walls and merging into laterally adjoiningnarrow-side adjusting regions where the broad-side walls are closertogether than in the widening region; the contour of the mold cavity inthe widening region is formed, as seen in sectional planes taken atright angles to the longitudinal axis of the mold, by curves which atleast in first partial regions form clothoids or clothoidal curves suchthat within the clothoids or clothoidal curves, the sudden changes incurvature are minimized or reduced to zero.
 2. The open-ended mold asclaimed in claim 1, wherein the curves include second partial regionsbetween successive first partial regions, and the second partial regionsare either formed approximately or exactly by arcs of a circle orstraight lines and are joined by the first partial regions of thecurves, whereby sudden changes in curvature (Δk) of these curve portionsat their transitional points are minimized or reduced to zero.
 3. Theopen-ended mold as claimed in claim 1, wherein the contour of the moldcavity in the widening region is formed in sectional planes, taken atright angles to the longitudinal axis of the mold, and the curves runsymmetrically in relation to the narrow-side walls with a convex-concaveprofile, the profile starting from the center of the mold cavity contourwith only a small curvature or no curvature, and going over into aregion with constantly increasing curvature.
 4. The open-ended mold asclaimed in claim 3, wherein the curvature (k) in the center of the moldcavity contour meets the condition k≦0.3*(4H)/(H ² +L ²), wherek=curvature (mm) H=half the mold cavity widening (mm) L=half the widthof the widening region B₁ (mm).
 5. The open-ended mold as claimed inclaim 1, further comprising transitions between the central region andthe narrow-side adjusting regions, wherein sudden changes in curvatureat the transitions from the central widening region to the narrow-sideadjusting regions and at locations of inflectional tangents in the curveprofile are minimized to tend toward zero.
 6. The open-ended mold asclaimed in claim 5, wherein the sudden changes in curvature (Δk) at thetransition from the central widening region to the narrow-side adjustingregions meet the condition Δk≦0.5*(4H)/(H ² +L ²) where k=curvature (mm)H=half the mold cavity widening (mm) L=half the width of the wideningregion B¹ (mm).
 7. The open-ended mold as claimed in claim 5, whereinthe sudden changes in curvature (Δk) at the locations of theinflectional tangents meet the condition Δk≦0.4*(8H)(H ² +L ²) wherek=curvature (mm) H=half the mold cavity widening (mm) L=half the widthof the widening region B₁ (mm).
 8. The open-ended mold as claimed inclaim 1, wherein the mold cavity in the widening region has the contourthereof, as seen formed in sectional planes taken parallel to thelongitudinal axis of the mold and the contour of the narrow-side walls,by straight lines between the mold inlet edge and the mold outlet edge.9. The open-ended mold as claimed in claim 1, further comprisingtransitions from the central widening region to the laterally adjoiningnarrow-side adjusting region regions, the transitions are formed by astraight line which, in a sectional plane, taken parallel to thelongitudinal axis of the mold and the narrow-side walls, joins the moldinlet edge and the mold outlet edge.
 10. The open-ended mold as claimedin claim 5, wherein in the contour of the mold cavity, in sectionalplanes taken at right angles to the longitudinal axis of the mold, thelocations of the inflectional tangents form straight lines between themold inlet edge and the mold outlet edge and the straight lines lie in asectional plane taken parallel to the longitudinal axis of the mold. 11.The open-ended mold as claimed in claim 8, wherein the straight linesare arranged inclined by the extent of strand shrinkage of the strandbeing molded.
 12. The open-ended mold as claimed in claim 1, wherein insectional planes taken parallel to the longitudinal axis of the mold andthe narrow-side walls, the contour of the mold cavity in the wideningregion has a curved profile between the mold inlet edge and the moldoutlet edge which is dependent on the solidifying conditions of themetal to be cast.
 13. The open-ended mold as claimed in claim 12,further comprising transitions between the central widening region andthe laterally adjoining narrow-side adjusting region are formed by acurve with an essentially parabolic profile and, in a sectional planeparallel to the longitudinal axis of the mold and the narrow-side walls,that curve joins the mold inlet edge and the mold outlet edge.
 14. Theopen-ended mold as claimed in claim 12, wherein in sectional planestaken at right angles to the longitudinal axis of the mold, the mold hasinflectional tangents at locations in the contour of the mold cavitywhich form curves having an essentially parabolic profile between themold inlet edge and the mold outlet edge, and the curves lie in asectional plane parallel to the longitudinal axis of the mold.
 15. Theopen-ended mold as claimed in claim 14, wherein the curves with theessentially parabolic profile are arranged inclined by the extent ofstrand shrinkage.
 16. The open-ended mold as claimed in claim 1, whereinin the widening region, in sectional planes taken at right angles to thelongitudinal axis of the mold, the radii of curvature of the contour ofthe mold cavity become greater between the mold inlet edge and the moldoutlet edge in the direction of the mold outlet edge until they areinfinite at the outlet edge.
 17. The open-ended mold as claimed claim 1,wherein the broad-side walls are essentially parallel to one another inthe narrow-side adjusting regions laterally of the central regions. 18.The open-ended mold as claimed in claim 1, wherein the narrow-side wallsare convexly formed with respect to the cavity in dependence on theshrinking behavior of a respective metal to be cast.
 19. The open-endedmold as claimed in claim 1, wherein the mold is shaped for casting ametal strand with a maximum thickness of 150 mm.
 20. The open-ended moldas claimed in claim 12, wherein the contour of the mold cavity in thewidening region has a curved parabolic profile.